In accordance with pending U.S. patent application Ser. No. 37,877, filed on May 10, 1979, in the name of John A. Horsley, et al, it is taught that isotopic selectivity can be increased in a laser isotope separation process by adjusting the temperature of a vapor which is to be irradiated such that a substantial portion of the molecules require more than one photon to dissociate. More particularly, it is specifically taught that the maximum separation factor which can be achieved with such molecules starting at thermal equilibrium is a function of the number of and the selectivity of the molecules requiring the least number of photons to dissociate from thermal equilibrium. The minimum number of photons which are necessary to produce dissociation thus depends on the energy gap between the highest thermally populated state and the lowest dissociative state. Thus, if the energy gap is equal to or less than one photon, energy selectivity will approach a single photon selectivity .alpha., where .alpha. is defined as the ratio of the small signal absorption cross-sections of the isotopic species at a given wavelength.